The present invention relates to a development apparatus for electrostatographic printing machines. More specifically, the present invention relates to a particular composition for a ceramic coated donor roll for use in a hybrid scavengeless development apparatus.
Generally, the process of electrostatographic reproduction includes uniformly charging a photoconductive member, or photoreceptor, to a substantially uniform potential, and imagewise discharging it or imagewise exposing it to light reflected from an original image being reproduced. The result is an electrostatically formed latent image on the photoconductive member. The latent image so formed is developed by bringing a charged developer material into contact therewith. Twocomponent and single-component developer materials are commonly used. A typical two-component developer material comprises magnetic carrier particles, having charged toner particles adhering triboelectrically thereto. A single component developer material typically comprises charged toner particles only. In either case, the charged toner particles when brought into contact with the latent image, are attracted to such image, thus forming a toner image on the photoconductive member. The toner image is subsequently transferred to a receiver sheet which is then passed through a fuser apparatus where the toner image is heated and permanently fused to the sheet, thus forming a hard copy of the original image.
To develop a latent image in an electrostatographic reproduction machine as above, charged toner particles either alone (single component), or mixed (two-component), are brought, by a development apparatus, into contact with the latent image formed on the photoreceptor. For two-component development, developer material containing carrier particles and toner particles is used. The development apparatus for such development typically includes a housing defining a chamber within which the developer material is mixed and charged. Moving and mixing two-component developer material triboelectrically and oppositely charges the carrier particles and the toner particles causing the toner particles to adhere to the carrier particles.
As disclosed for example in U.S. Pat. No. 5,245,392, and U.S. Ser. No. 07/091,858 both assigned to the assignee of the present application, one type of a two-component development method and apparatus is referred to as "hybrid scavengeless development", and is very suitable for image-on-image development type processes. The apparatus includes a housing defining a development zone, and a mixing chamber holding developer material containing carrier and toner particles. The apparatus also includes a developer material transport roll and a donor member such as a donor roll for receiving charged toner particles from the developer material transport roll and transporting them to the development zone. A plurality of electrode wires are embedded in, or are closely spaced relative to, the donor roll within the development zone. An AC voltage is applied to the electrode wires for forming a toner cloud in the development zone. Electrostatic fields generated by an adjacent latent image on a photoreceptor surface serve to attract charged toner particles from the toner cloud, thus developing the latent image.
Single component development systems, referred to as jumping gap development, can also use a donor roll for transporting charged toner particles directly from a toner chamber to the development zone. The charged toner particles similarly are attracted by and develop an electrostatic latent image recorded on a photoconductive surface. In jumping gap development, an AC voltage is applied to the donor roll for detaching toner particles from the donor roll and projecting them toward an adjacent photoconductive surface holding the electrostatic latent image.
In either of the above discussed development systems for example, the donor member or roll and its electrical and chemical characteristics are very important to the ability of the development apparatus repeatably transport acceptable and uniform quantities of toner particles into the development zone, as well as effective support the electrostatic fields necessary within the development zone for high quality image development. For example, the donor roll must be suitable for charged toner particles to effectively and controllably (even at high speeds) adhere electrostatically thereto. The surface of the donor roll must be partially conductive relative to a more conductive core, and this partial conductivity on the surface should be uniform throughout the entire circumferential surface area. The range of conductivity of a donor roll should be well chosen in order to maximize the efficiency of a donor roll in view of any number of designed parameters, such as energy consumption, mechanical control and the discharge time-constant of the surface thereof.
In image-on-image type processes with a pre-developed toner image already on the photoreceptor, the donor roll should also act as an electrostatic "intermediate" between the photoreceptor and the developer transport roll in order to minimize unwanted interactions between the development system and the photoreceptor. Minimizing such interactions is particularly desirable in such processes because the single photoreceptor therein is to be charged, exposed and developed several times usually in a single, as in single pass highlight color process or in a single pass full color process.
The donor roll must further have desirable wear-resistant properties so that the surface thereof will not be readily abraded by adjacent surfaces. Further, the surface of the donor roll should be without anomalies such as pin holes, which may be created in the course of its manufacture. Pinholes created in the manufacturing process or abrasions caused in its use, can result in electrostatic "hot spots" and undesirable electrical arcing in the vicinity of such structural imperfections. Ultimately, the most important requirement of the donor roll can be summarized by the phrase "uniform conductivity;" Other physical properties of the donor roll, such as the mechanical adhesion of toner particles, are also important, but are generally not as quantifiable in designing development apparatus.
As disclosed for example in each of the following references, particular attempts have been disclosed for providing donor rolls with specific features and characteristics towards meeting some of the requirements as stated above.
For example, U.S. Pat. No. 3,950,089 discloses a development apparatus in which a surface for the direct conveyance of electrically-conductive toner comprises a dielectric sheath of a thickness of 1-25 mils, having a resistivity of 10.sup.7 to 10.sup.9 ohm-cm.
U.S. Pat. No. 4,034,709 discloses a development apparatus in which asurface for the direct conveyance of toner comprises styrene-butadiene, of a resistivity of 10.sup.2 to 10.sup.6 ohm-cm.
U.S. Pat. No. 4,774,541 discloses a development apparatus in which a surface for the direct conveyance of toner is doped with carbon black to a conductivity of 10.sup.-6 to 10.sup.-10 1ohm-cm.
Co-pending application Ser. No. 07/955,965, filed Oct. 2, 1992, discloses a phenolic resin coated on a donor roll. The use of phenolic resin coated donor rolls results in discharge time constants less than 300 microseconds.
In the prior art, there are a few instances in which the physical properties of ceramics are exploited for various purposes relating to development of electrostatic latent images.
U.S. Pat. No. 4,544,828 discloses a heating device utilizing ceramic particles as a heat source and adapted for use as a fixing apparatus.
U.S. Pat. No. 4,893,151 discloses a single component image developing apparatus including a developing roller coated with a Chemical Vapor Deposition ceramic and an elastic blade coated with a ceramic.